Watering in the dark

By
Ed Rosenthal
on July 2, 2001

Is it necessary to water my plants in sleep time? Do they feed only when the lights are on?
Ozzie,
Elizabeth Vale, South Australia

Plants do not shut down at night. Although photosynthesis stops because there is no energy to power it, plants continue other metabolic processes at night. These include tissue building and respiration, the life process in which cells use energy in order to live. These processes are powered by sugar. During the light period, the plant produces more sugar than it uses for metabolism. This sugar is used during the dark cycle for energy. To build tissue, plants combine nitrogen with sugar to form amino acids, the building blocks of protein.

As the sugar is used to fuel the life processes, plants absorb oxygen from the air and release CO2. Water is used for both metabolism and tissue building, although in much smaller quantities than when the plants are photosynthesizing.

The root environment is of concern as well. As water evaporates it leaves behind the soluble salts so there is a buildup of nutrients at the surface.

An irrigation every four hours during the dark period provides fresh water for the roots to draw upon. At the same time, it rinses excess nutrients from the medium surface.

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